Calling a constructor inside a constructor of a templated class

Question

This code has a templated class. The default constructor appears to call itself recursively. How can it do that? I don't understand this code. Maybe if I would be given an example without templates, just POD types, things would be clearer. I haven't encountered this construct before in C++ programming. I think that I don't understand both the constructor and the templates.

template <typename T>
class Simple {
  public:
    Simple(T value = T());    // What's this?
    T value();
    void set_value(T value);
  private:
    T value_;
};

template<typename T>
Simple<T>::Simple(T value) {
  value_ = value;
}

template<typename T>
T Simple<T>::value() {
  return value_;
}

template<typename T>
void Simple<T>::set_value(T value) {
  value_ = value;
}

My question is: What does T value = T() do?

Answer 1

Class Simple has a variable value of type T (Templated).

The constructor which you are pointing is a default constructor. When no parameter is supplied while creating Simple object. Then default constructor will instantiate the value object to the default constructor of T.

Either , Simple(T value = T()) is a default constructor which is instantiating value to default constructor of typed element.

Example :- if T is String.

Simple (String value = String()) 

so value is now initialized to default of String().